A boat drive of this type is also called a stern drive or rear drive. In a stern drive an internal combustion engine is arranged on the inside of a vessel's hull and the drive-train passes through a transom of the hull, so that the remainder of the drive-train, in the form of a drive unit, is outside the hull. To enable steering of the travel direction the drive unit can be pivoted about an approximately vertical steering axis. The propeller shaft of the drive unit then also swivels and the thrust vector produced by the rotating propellers changes its direction, whereby the travel direction of the boat also changes.
Exhaust-gas systems for stern drives are known, which are cooled with cooling water. In such cases the cooling water is passed into the exhaust-gas system and at least in part discharged together with the exhaust gases below the surface of the water. For example, the exhaust gases and the cooling water pass through an exhaust-gas duct through the boat drive to a propeller shaft arranged on the drive output side, and are discharged through an exhaust-gas outlet in the area of the propeller shaft below the surface of the water. This reduces annoying noise emissions and prevents unpleasant effects of the exhaust gases for people on the boat.
The passage of the exhaust gases and the cooling water through the relatively long exhaust-gas duct and through the exhaust-gas outlet below the water surface give rise to a high flow resistance against the exhaust gas stream and therefore have the disadvantage that an undesired exhaust-gas counter-pressure acts upon the internal combustion engine. This reduces the drive power that can be provided by the internal combustion engine. To overcome this disadvantage DE 3542557 A1 describes a boat drive with an exhaust-gas system in which the counter-pressure of the exhaust gas is relieved, in that the exhaust gases do not pass through the whole of the exhaust-gas duct and the outlet located below the water's surface, but are instead discharged to the outside air through an exhaust-gas release duct above water level.